Purpose: The inter-fraction motion of the rectum for patients with rectal cancer is not well studied. Using daily kilo-voltage cone beam CTs, the purpose of this study is to quantify the interfractional rectal motion relative to the planning CT and to assess proper planning margins for these patients.

Methods: A total of 211 daily CBCT from 9 patients, who received IMRT treatment, were selected for this study. On each CBCT, the treatment target volume of the rectum was retrospectively contoured. For each patient, the daily contoured rectum (25 CBCT/patient on average) was transferred to the planning CT with a rigid bone registration. The planning margins to compensate for the interfractional rectal motion were determined by expanding the planning rectum (clinical target volume) with a series of margins up to 2.0 cm. The daily rectal motion was measured in superior, middle, and inferior segments. For each segmented rectum, the lateral and anterior-posterior motions were measured relative to the centroid of the corresponding segmented CTV.

Results: For all patients, 1.7 cm uniform expansion of the CTV was required to achieve a geometric coverage in 96% fractions. Furthermore, 1.7cm uniform expansion resulted in geometric coverage in 99% fractions of the first half course of the treatment and in 92% fractions of the second half course. There is a direct correlation between the segment location and magnitude of the displacement/movement (r=0.99). For these patients, the average margin expansions required for 100% geometric coverage for the Inferior, Middle, and Superior segments of the rectum were 0.8 cm, 1.0 cm, and 1.2 cm respectively.

Conclusion: We found that the superior segment of the rectal wall is more mobile than the other segments, followed by the middle and inferior segments. Non-uniform margins may be considered to ensure adequate geometric coverage while sparing surrounding normal tissue.